Big-Block Bumpstick Blowout

Testing cam profiles in a 454

01. We had on hand a GM Performance Parts 454 HO crate engine, a mill that is mild in compression ratio at 8.6:1, but features large rectangular port cylinder heads. The basic combo is similar to the 454s used in big-block Corvettes in the early '70s. What differences could be found in cam selection?

Ah, the camshaft: that magic stick that offers the promise of so much extra horsepower, while at the same time creating the fear of transforming a tractable and smooth powerplant into a snorting, lumping, and bumping beast. For some, the mechanical melody of a stout stick is sweet music, with the menacing rat-tat-tat of a long-duration lope being just the prescription for boulevard presence. Others prefer a subtler scheme, stepping up the specification for a gain in mechanical muscle, but not at the expense of down-the-road practicality. The choices in camshafts are virtually endless. With so many possible permutations, where does one begin in selecting a camshaft--be it for a cruiser or bruiser? It helps to know the specifications, and how they play in the engine's operation.

It's no secret that the camshaft profile has a key effect on engine output. An engine produces power largely by virtue of airflow, and flow into and out of the cylinders is via the valves. The camshaft's duty is to provide for the opening and closing of the valves, a function critical to power production. Stripped to its basics, the camshaft's specifications relate to valve action in terms of duration and lift. "Lift" is simple enough to understand, referring to how far the valve is opened off the seat, with the specification provided as a fraction of an inch.

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02. Topping the big-block was a Performer RPM intake manifold from Edelbrock, with an 850 Speed Demon carburetor.

Duration, the measure of how long the valves are held open, needs a reference in order to be quantified. To express this value of "how long," camshaft duration is given in degrees of crankshaft rotation, essentially measuring how far around the crank turns in degrees during the period a cam lobe is lifting the lifter. Duration is checked at a given specification, with "advertised" duration numbers recording the open time from a nominal lift value, such as .008 inch, while another standard is the commonly used duration at .050 inch.Why is it so important to know about duration when selecting a cam? Duration has a dramatic effect on the idle quality and rpm range of the engine. In general, longer-duration cams will raise the rpm at which peak torque will occur, and raise peak horsepower. The drawbacks are losses in low-rpm cylinder pressure, torque, and idle quality. Therein lies the tradeoff in cam selection: go bigger for more bite up top, but too big and the engine will become increasingly hard to live with.

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03. A set of Hooker 2-inch headers was utilized for testing purposes, run with collector extensions and open exhaust.

Revisiting lift: As with duration, stepping it up will generally increase output, and, to a lesser extent rpm capabilities, provided the cylinder heads, intake, and exhaust system are up to the task. However, higher peak lift doesn't come with as pronounced a penalty in low-engine speed operation. Here lies an opportunity to gain a performance advantage.

If the camshaft is designed to open the valves at a more aggressive rate, the lobe can provide more lift at a given duration level. Competition Cams sought to exploit these possibilities with its aggressive Xtreme Energy line of camshafts. These faster than traditional lobe profiles accelerate more quickly and at a higher velocity. The result is the power-enhancing benefit of higher lift while mitigating the effects of excessive duration. Bottom line: It means more power and low-end torque for a given cam-duration "size."

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07. With our baseline Comp XE268H cam installed, we had a stout 483hp on tap, in a relatively mild-mannered package. Less duration generally means less temperamental running at idle and lower rpm.

To get an idea of how cam size relates to output, the pros and cons, trade-offs and gains, we thought we'd see for ourselves how moving up or down the camshaft ladder affects an engine's output. We brought a GM Performance Parts HO 454 crate engine to Westech Performance Group's test facility in Southern California to do some testing. Our engine was based on a Gen V block, with a modest 8.6:1 compression ratio, topped with rectangular-port iron heads, and is rated at 425 hp. The only upgrade to the package was the addition of Edelbrock's excellent Performer RPM intake manifold, which we topped with an 850 Speed Demon carburetor. Along with the engine, we brought along a range of Comp's Xtreme Energy camshafts. We'll have a look at the power numbers, record the data, and we'll leave it up to you to decide which level of performance is right for your application.

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08. Swapping the cam begins with removing the valve covers to clear the way.

We started with Comp's popular XE268 grind, a powerful street performance camshaft we have found to offer good driveability, even in daily-use applications with air conditioning and full accessories. This would be the baseline setup, with the plan being to step up the range from there, and gauge the effects. With the cam installed and run in on the dyno, we prepared for the test. The engine settled into a smooth idle at 850 rpm, with just enough grumble to distinguish the test engine as a performance mill. True to form, the XE268 showed very good output, posting 483 hp at 5,300 rpm, and 541 lb-ft of torque at 3,900 rpm. The relatively mild-mannered idle quality and excellent torque lower in the rpm range make the XE268 one of our favorite street grinds. The fast valve action produces output up top that would normally be seen only from a choppier, longer-duration cam of more conventional ramp rates. Testing on our 454 showed 15.7 inches Hg of manifold vacuum at idle, an exceptional level for an engine of this output. If the goal is getting the best of both worlds in a street and strip cam, the XE268 is one of our favorite choices.

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09. The distributor needs to come out; in our case we were running a new MSD billet unit, a piece we find quite accurate in its timing, as well as easy to adjust.

In short order, we tore our test engine back down and stabbed in the next size up in the Comp XE line: the 274. The 274 adds 6 degrees of duration at 0.050 inch, bringing it up to 230 degrees. This level of duration is getting toward the edgy side for a true street machine, particularly in an otherwise stock car with an automatic and A/C. To get the most from this cam, an aftermarket higher-stall converter should be part of the program, though four-speed cars will be fine. As expected, the added duration resulted in a decline of idle vacuum, with our instrument now reading 12.4 inches Hg at the same 900-rpm idle. Power was up significantly, with output boosted to 507 hp at 5,700, though peak torque actually dropped slightly to 538 lb-ft at 4,000 rpm. Essentially, the result of the bigger cam was a shift in the torque curve, with a drop in torque in the rpm range below peak torque, and a corresponding gain in torque in the rpm range above.

It's the higher rpm torque that makes for the higher peak-horsepower output. The large displacement of the 454, with its large rectangular-port cylinder heads, certainly responded favorably to the more generous valve events effectuated by the XE274.

We swapped up one last time, this time moving up to the XE284, not the biggest cam in the XE line, but about as radical as we would consider in a vehicle expected to do any real street time. Make no mistake about it, the 284 is a serious stick, which should be reserved for more serious applications. With the 284, the idle quality changed to a distinct lope, and idle vacuum dropped to 9.5 inches Hg. This cam is best used in a combination designed to complement the higher rpm range in which it's most at home, including gear, converter, and compression. In fact, the recorded drop in cranking compression indicated that our test engine could tolerate a good measure higher compression ratio with this cam. Power accessories can be made to work with sharp ignition tuning and tricks such as vacuum reserve systems for power brakes, throttle jacks system for the A/C, and the like; however the 284 is most in its element in a stripped-down package. Not surprisingly, power at high rpm was up significantly.

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